The invention relates to a circuit for monitoring the voltage and current of individual cells of a multi-cell battery during charging and to bypass charging current in excess of a pre-set value that can be varied.
Multi-cell, rechargeable batteries, such as those of the lithium ion type, are often used in mission control applications, such as uninterruptible power supplies and various military applications. The output voltage of such batteries depends on the numbers of cells connected in series and the particular chemistry selected for the cells. In some applications, a sufficient number of cells can be connected to achieve voltages as high as 400V.
As a multi-cell battery is being charged or recharged, a current source is connected across all of the series connected cells. As the charging takes place, individual cells might react differently to the charging current. In particular, it is desired that a cell not be overcharged since this would damage the cell and perhaps even the battery. Various circuits have been used to bypass excess current from reaching an individual cell during a charging cycle so that it will not be damaged. It is also desirable to monitor the state-of-health of each of the battery individual cells and the composite battery. This involves determining such parameters as the internal resistance, polarization resistance, and remaining capacity of each cell of the battery as a percentage of original capacity measured in ampere-hours, often called the state-of-charge (SOC).
The present invention relates to a circuit that can monitor and protect individual cells of a multi-cell battery from over-charge and acquire data parameters to be used to determine various characteristics of the cell state-of-health.
The circuit in accordance with the invention is connected to each individual cell of a multi-cell battery to be monitored as the battery is being charged. The circuit includes a portion to bypass charging current from the cell, this portion being pre-settable in a variable manner to bypass current above a desired high voltage limit for the cell. As the battery is being charged, the bypass circuit will shunt current around a cell when the pre-set voltage level is exceeded, thus preventing any damage to the cell.
In another aspect of the invention, the circuit can be operated to produce a pulse of discharge current. When this is done the change in voltage of the monitored cell in response to the change in current, dV/dl, can be used to determine the cell internal resistance. The cell polarization resistance also can be determined by extending the discharge pulse. The data acquired can be used to determine factors relating to the state of charge (SOC) of a cell and its state of health.
An object of the invention is to provide a circuit to monitor all individual cells of a multi-cell battery during its operation.
An additional object is to provide a circuit to monitor an individual cell of a multi-cell battery during charging and to bypass charging current if a pre-set upper limit of the cell is exceeded during battery charging while permitting the charging of the other cells which have not reached the pre-set voltage limit.
Another object is to provide a monitoring circuit for individual cells of a multi-cell battery in which, as a cell is added to a battery pack, a monitoring circuit for the cell also is added in a modular fashion and interfaced to a controller in a modular fashion.
Yet another object is to provide a circuit to monitor individual cells of a multi-cell battery that can be operated to acquire data of a cell that is indicative of its state-of-health and state-of-charge.
Still a further object is to provide a monitoring circuit for each cell of a multi-cell battery that protects the cell against overcharge by bypassing current after the cell is charged to a pre-set upper voltage limit and that can be operated to acquire data to be used to determine the cell state-of-charge and state-of-health.